1. Field of the Invention
This invention relates to a developing roller and apparatus for use in electrophotographic and electrostatic recording machines such as copiers and printers for developing an electrostatic latent image with a developer such as a one-component developer for visualization. More particularly, it relates to a developing roller and apparatus which is improved in anti-staining so that acceptable images can be produced in a stable manner over a long term.
2. Prior Art
In conjunction with prior art electrophotographic and electrostatic recording machines such as copiers and printers, one typical developing method is an impression developing method as disclosed in U.S. Pat. Nos. 3,152,012 and 3,731,146. This developing method visualizes electrostatic latent images by supplying a non-magnetic one-component developer to a photoconductor drum bearing a latent image, thereby adhering the developer to the latent image. Since the impression developing method eliminates a need for magnetic material, the apparatus can be simple and compact and color toner can be used.
More particularly, the impression developing method is by bringing a developing roller carrying a toner or non-magnetic one-component developer in contact with a latent image holder or photoconductor drum bearing an electrostatic latent image, thereby adhering the toner to the latent image. The developing roller must be formed of an elastic material having electrical conductivity.
Referring to FIG. 2, the impression developing method is briefly described. A developing roller 1 having a conductive elastomer layer is placed between a toner feed roller 4 for feeding a toner 6 and a photoconductor drum 5 having an electrostatic latent image borne thereon. Upon rotation of the developing roller 1, photoconductor drum 5, and toner feed roller 4 in the directions shown by arrows, the toner 6 is fed from the feed roller 5 onto the surface of the developing roller 1 and 15 regulated into a uniform thin layer by a doctor blade 7. The thin layer of toner is then delivered from the developing roller 1 to the photoconductor drum 5 to adhere to the latent image whereby the latent image is developed into a visible toner image. The toner image is finally transferred from the photoconductor drum 5 to a record medium, typically paper in a transfer section 8. Also included is a cleaning section 9 having a cleaning blade 10 for scraping off the toner left on the photoconductor drum 5 after the transfer step.
During rotation, the developing roller 1 must maintain close contact with the photoconductor drum 5. The conventional developing roller 1 is of a structure having a conductive elastic layer 3 around a shaft 2 as shown in FIG. 1. The shaft 2 is of a highly conductive material, typically metal. The conductive elastic layer 3 is formed of a conductive elastomer in the form of an elastic rubber such as silicone rubber, acrylonitrile-butadiene rubber (NBR), and ethylene-propylene-diene terpolymer (EPDM) or a sponge such as urethane foam, with a suitable conductive agent being blended therein.
However, the prior art developing rollers had the following problems resulting from the properties of elastic layers used therein.
(1) Where the developing roller is formed of an elastic rubber such as silicone rubber, NBR and EPDM having a low hardness enough to achieve tight contact, the photoconductor drum can be contaminated therewith.
(2) Where a spongy body such as urethane foam is used to form the elastic layer, toner can penetrate into pores in the elastic layer. As the penetrating toner accumulates on long-term use, the roller becomes harder and electrical charging of toner becomes inefficient, resulting in a drop of image quality. This tendency is also found when the roller has a surface receptive to toner deposits.
(3) Where the developing roller has a single elastic layer, it creates a greater burden between the developing blade and the photoconductor because of a very large coefficient of friction on its surface. This leads to uneven toner transfer and driving jitter, both resulting in defective images. Also in the developing roller with a single elastic layer, charging of toner on the roller does not show a quick rise and the roller surface is readily contaminated with toner. This often invites fogging due to poor charging of toner and a loss of print density. A substantial drop of print quality occurs after long-term continuous printing.
Therefore, an object of the present invention is to provide a developing roller which has a relatively low hardness enough to ensure tight contact and does not contaminate the photoconductor drum so that images of quality free of density variation and background fog can be consistently produced over a long term without a drop of quality. Another object of the present invention is to provide a developing apparatus using such a developing roller.